The present invention relates to satellite communications systems. More specifically, the invention relates to a method and apparatus for virtual path asynchronous transfer mode switching in a processing satellite communications system.
Asynchronous Transfer Mode (ATM) is a cell-based switching and multiplexing technology which may be used as a general-purpose, connection-oriented transfer technique for a wide range of services. The primary unit of transfer in ATM is a cell (also referred to as an “ATM cell”). Each ATM cell is a specifically formatted data unit that is 53 bytes long and includes 48 information bytes (referred to below as the “payload”) and 5 header bytes (called the “cell header”). The primary function of the cell header is to carry the address of the cell. The header includes two individual fields known as the Virtual Path Identifier (VPI) and the Virtual Channel Identifier (VCI), which jointly serve as the “addressing field.” The VPI and VCI fields are comprised of 12 address bits and 16 address bits, respectively, providing a total of 28 bits of address.
The process of switching an ATM cell at an ATM switch element generally includes two steps: a routing step and a relabeling step. During the routing step, an output port of the switch is identified for each cell that arrives at an input port of the switch. In present ATM systems, large routing tables at each connecting point (i.e., node) along the virtual connection (explained below) are generally used to map the VPI and VCI from the input ports to the output ports of the ATM switch. The routing step may be performed, for example, by first indexing a particular memory location (as determined by part or all of the addressing field) in one of the routing tables. The routing information stored at the indexed memory location generally holds a routing tag which indicates the next link for the current ATM cell. This routing tag may then be attached to the current ATM cell, to identify that cell by its next link.
The second step in ATM switching is a relabeling step. Generally, the VPI and VCI only have “local significance.” In other words, the VPI and VCI are only significant for one particular link of the virtual connection. Therefore, in order for accurate switching to occur at the next link, the VPI and possibly the VCI must often be changed or relabeled to provide a new address, so that the new address may be understood at the next link.
Generally, two levels of routing hierarchies, Virtual Paths (VP) and Virtual Channels (VC), are defined for ATM traffic. A VC link is defined as the unidirectional transport of ATM cells with the same VCI between a VC connecting point (i.e., a node) and either a VC endpoint (i.e., the origination or destination) or another VC connecting point. A VP describes the unidirectional transport of ATM cells belonging to a group of VCs that are associated with a common VPI. Each VP has a bandwidth associated with it limiting the number of VCs that may be multiplexed on a VP. A VP link provides unidirectional transfer of cells with the same VPI between a VP connecting point and either a VP endpoint or another VP connecting point.
One of two types of virtual connections between an ATM cell's origination and destination (i.e., between endpoints) is generally formed prior to data flow. This logical connection is generally established based on information in the cell headers, including the VCI and VPI. A Virtual Channel Connection (VCC) is a concatenated ordered list of VC links from one user to one or more other users. In general, the VCI in the cell header identifies a single VC on a particular VP. Similarly, a Virtual Path Connection (VPC) is a concatenated ordered list of VP links from one or more users to one or more other users. Thus, while a VCC or VPC exists between endpoints, a VC or VP may exist between an endpoint and a connecting point or between connecting points.
Generally, two types of switching are recognized in ATM. One type of switching is VP switching, where only the VPI is considered in the switching action, and the VCI is ignored. In this type of switching, the VPI alone is sufficient to determine, for example, an appropriate memory location in the routing tables. Here, relabeling of the VPI is allowed, but the VCI must remain unchanged. A second type of switching is VC switching, where the combination of the VCI and VPI is significant in both the routing and relabeling steps.
Today, satellite systems commonly transmit and receive communications signals to and from user terminals. Such satellite based communications systems may utilize one satellite or a constellation of several satellites to relay communications signals to and from the user terminals (fixed or mobile). Each satellite includes at least one antenna which defines the satellite's coverage region on the earth called its footprint. The satellite antenna(s) may divide the footprint into multiple beam spots, or the footprint may be a single beam spot. Each beam spot is assigned one or more frequency bands (subbands), in which communications signals travel between the satellite and each user terminal.
The user terminals communicate along preassigned communications channels in the subband. The transmission of information along a preassigned communications channel to a satellite switch occurs on the uplink, and the transmission of information along a preassigned communications channel from a satellite switch occurs on the downlink.
Current processing satellites typically transmit data in a “bent pipe” fashion where uplink transmissions are simply retransmitted on the downlink. In another method used by processing satellites to transmit data, uplink transmissions may be demodulated and decoded at the satellite to recover one or more ATM cells that were conveyed across the uplink transmission path (TP). The ATM cells are then switched onto the downlink TP serving the destination earth terminal to which the cell is directed. In addition, processing satellite systems may incorporate crosslinks which link one satellite to another. These crosslinks are analogous to inter-switch transmission paths in terrestrial ATM networks.
In satellite communications systems which use ATM, a virtual connection is generally established through the network from origination to destination, where ATM cells are routed over the same set of TPs for the duration of a call. As explained above, the virtual connections may be, for example, a Virtual Path Connection (VPC) or a Virtual Channel Connection (VCC) from origination to destination. Although ATM has been applied to satellite communications systems in the past, ATM switching has not been implemented in processing satellites.
When applying past ATM methods to processing satellite communications systems, several problems arise. First, the size of the routing tables needed to support VC switching is extremely large, thus greatly increasing the memory storage capacity requirements on the satellites. An increase in the required memory leads to an increase in both the physical size and the cost of communications satellites. Furthermore, this additional memory must be robust to protect against logic bits that are altered as a result of high energy particle bombardment. This radiation hardening further increases the cost of communications satellites.
A second disadvantage associated with past ATM switching applied to processing satellite communications systems is that VC switching (which is typically the type of ATM switching that is applied) would require a substantial amount of control traffic between a terrestrial network control center (NCC) and the satellite for each VCC that is set up or torn down by the system. The setting up and tearing down of VCCs occurs quite frequently in a large scale system such as a processing satellite system. Thus, the presence of frequent bursts of control traffic between the NCC and satellites greatly reduces the bandwidth capacity available in the uplinks and downlinks for transmission of substantive information. Further, the existence of control traffic introduces additional failure modes that may result due to lost or redirected cells bearing control information. In addition, the hardware required to process this control traffic increases the overall size and cost of the communications satellites.
A third disadvantage associated with standard ATM switching in a processing satellite communications systems is that the relabeling of the VPI and VCI typically required for VC switching generally constitutes additional processing in the cell switching process. Again, this results in additional hardware requirements which leads to an increase in the size and cost of the communications satellites.
A need exists in the satellite communications industry for a low cost and efficient method and apparatus for virtual switching in a beam spot processing satellite communications system.